This specification relates to a resonator device for magnetic resonance applications, including electron spin resonance (ESR) applications. Magnetic resonance systems are used to study various types of samples and phenomena. In some magnetic resonance applications, the spins in a sample are polarized by a static, external magnetic field, and a resonator manipulates the spins by producing a magnetic field at a frequency near the spins' resonance frequencies. In electron spin resonance (ESR) applications, resonators typically operate at microwave frequencies to interact with electron spins in the sample.